1. Field of the Invention
This invention broadly relates to an improved phenolic resin composition useful in adhesives for making composite wood products, particularly plywood. The invention also relates to a process for making plywood.
2. Description of Related Art
Phenolic resin-based adhesives are well-known for their exceptional strength and durability. Such resins have achieved wide spread acceptance in the United States and elsewhere as the principal component of adhesives used in making wood laminates such as plywood, particularly for exterior exposure. Phenolic resins for use in such adhesives generally are prepared by condensing phenol with a molar excess of formaldehyde under alkaline reaction conditions. The resulting resins, typically referred to as resoles, are thermosetting polymers or oligomers.
For producing plywood, the adhesive is applied to the mating surfaces of wood veneers and the veneers are subjected to a pressing operation at an elevated temperature to consolidate them into a unitary panel and to cure the adhesive. Often, the pressing is performed in two stages to maximize output; using a first or pre-press stage at ambient temperature and under a pressure and for a time just sufficient to consolidate the veneers into a unitary panel. The consolidated panel can be stored and handled without shifting or separation of the veneers and afterwards can be treated under heat and pressure to cure the thermosetting adhesive and form the final laminate.
One recognized draw back of conventional phenol-formaldehyde resins is their slow rate of cure.
In preparing wood laminates such as plywood with such adhesives, it has long been conventional to dry the wood veneers to a very low moisture content, i.e., to less than about 5% average moisture content (5 lbs. water/100 lbs. dry wood), before application of the adhesive and consolidation of the veneers. Use of high moisture content veneers with adhesives formulated with conventional phenolic resins invariably leads to a large number of lamination defects and an excessively large number of rejected panels because of poor lamination.
Many lamination defects are believed to be caused by steam formation between veneer layers and a blow out of the steam when pressure is released upon completion of the pressing cycle. As the temperature increases in the center of the veneers during consolidation, so does the vapor pressure of trapped steam. As the press is opened, the built-up vapor or steam seeks an avenue of escape and blows the panel.
Sizable operating and capital costs have been incurred in the prior art to assure consistently low moisture content for wood veneers used in making wood laminates such as plywood, and thus eliminate lamination defects and reduce the number of rejected panels. The prior art, however, has recognized the advantages to be gained in both operating and capital costs if higher moisture content veneers could be employed in the preparation of wood laminates, and the plywood industry recently has increased its efforts to identify adhesives suitable for bonding higher moisture content veneers.
In U.S. Pat. No. 4,239,577, for example, a process is described for preparing wood laminates from high moisture content wood veneers. In accordance with this process, panels are prepared using veneers of differing moisture contents with higher moisture content veneers constituting the outer layers of the consolidated panel and lower moisture content veneers constituting the panel core. While this process purports to ameliorate problems encountered when using high moisture content veneers with adhesives formulated with conventional phenolic resins, it does not eliminate the need for drying at least some of the veneers to a low moisture content.
In U.S. Pat. No. 4,412,945, an adhesive is described which purportedly permits the preparation of wood laminates using high moisture content veneers. The adhesive composition combines a phenol-aldehyde resin, an alkaline catalyst and from 5-50%, based on the weight of resin solids, of a bentonite clay. The adhesive also may contain other conventional additives such as fillers and extenders. Apparently, this adhesive has not received widespread acceptance in the plywood industry, which continues to search for ways to reduce substantially the need to dry high moisture content veneers before assembling them into unitary wood laminates.
Reduced veneer drying requirements result in savings in capital costs and processing time and also lead to improvements in the dimensional stability of the consolidated wood laminates. Conventional laminates made with veneers having less than about 5% moisture content tend to swell in size as the moisture content of the wood laminate increases to its equilibrium content gradually with time. In the summer months, the equilibrium moisture content of a wood laminate may be as high as about 10% by wt. Finally, the ability to prepare laminates using high moisture content veneers, e.g. about 7 wt. % and higher, also relaxes the criticality of moisture control in veneer preparation.
Through the years, various other modifications also have been proposed for producing phenol-formaldehyde resins which exhibit a faster cure rate without sacrificing bond strength and quality. Such resins might be better suited for bonding veneer of higher moisture content. Thus, the prior art has employed the use of more reactive phenols such as resorcinol, reactive additives such as formamide, or the addition of various cure rate accelerators such as potassium carbonate, just prior to use. Unfortunately, these approaches have not succeeded for a variety of reasons, including increased costs, inconvenience, and health and safety considerations.
Daisy et al. U.S. Pat. No. 4,758,478 indicates that by replacing at least a part of the sodium hydroxide commonly used to prepare phenol-formaldehyde condensates, with potassium hydroxide, a faster curing resin is obtained. The patent further asserts that such a potassium-modified resole resin exhibits improved cure speed without any loss in rheological properties. Consequently, reduced adhesive spreads are possible and it is possible to use such resins to prepare adhesives for making plywood from veneers of increased moisture content.
Unfortunately, potassium hydroxide is a more expensive source of alkalinity than sodium hydroxide and thus such resins are more expensive than their conventional counterparts. Thus, there remains a need in the art for methods for modifying conventional phenolformaldehyde resins to provide a more economical, faster curing resin suitable for bonding veneers with higher average moisture contents.
Applicant has found that it is possible to prepare a faster curing and moisture tolerant resole resin, while using sodium hydroxide to supply all of the alkalinity for preparing the phenolic resin, as has commonly been done in preparing phenol-formaldehyde resole resins, by also using a small amount of certain water-soluble potassium salts selected from the group consisting of potassium chloride, potassium sulfate and mixtures thereof. Quite surprisingly, the molecular weight distribution of such a potassium salt-modified phenolic resin is strikingly similar to resins prepared by using a combination of potassium hydroxide and sodium hydroxide.
Thus, resins of the present invention exhibit similar properties to those prepared using a combination of sodium hydroxide and potassium hydroxide but are much more economical due to the significantly lower cost for the water-soluble potassium salt additives and the typically lower amount of such salts which are required to obtain desirable resin properties relative to the amount of potassium hydroxide needed to obtain a similar resin. Resins of the present invention are particularly suitable for preparing adhesives used for making plywood, particularly plywood made from veneers having a relatively high average moisture content.